In the past, bifocal and trifocal lenses have been popular as eyeglass lenses for aiding the eyes' accommodation when the eye's accommodation ability has weakened. In addition, monofocal lenses for specific near-distances are also well known. In recent years, "progressive power" lenses have started to become common for use as eyeglass lenses.
A progressive power lens typically includes a far-vision corrective portion (also termed herein a "far-vision portion") usually located at the upper portion of the lens when the lens is being worn, a near-vision correction portion (termed herein a "near-vision portion") usually located at the lower portion of the lens when the lens is worn, and an "intermediate-vision portion" between the near-vision portion and the far-vision portion. The intermediate-vision portion comprises a "progressive zone" that defines a gradual transition in refractive power between the far-vision portion and the near-vision portion. I.e., in the progressive zone, the refractive power exhibits a relatively gradual change from the far-vision portion to the near-vision portion.
Progressive power lenses can advantageously at least partially compensate for decreased accommodation ability of a wearer's eyes by lowering the wearer's visual axis when the lenses are worn. Also, unlike conventional bifocal and trifocal lenses, progressive power lenses do not present any discernible dividing lines separating one portion from another on the surface of the lens.
The distance between the center of the far-vision portion and the center of the near-vision portion is referred to herein as the "length of the progressive-zone." The amount of change (usually an increase in refractive power impressed between the far-vision center and the near-vision center) is called the "additional power." In addition, as used herein, terms including "up" or "upper", "down" or "lower", "horizontal," and "vertical" indicate positional relationships in the lens when being worn.
Thus, conventional progressive power lenses comprise three portions: a far-vision portion, a near-vision portion, and an intermediate-vision portion, all formed on a single refractive surface having limited area. If a wide "distinct-vision area" (an area through which the wearer can see without experiencing blurred vision) is to be maintained in the far-vision portion and in the near-vision portion, and when the space between the two portions is defined by an intermediate-vision portion comprising a progressive zone, lens aberrations tend to be concentrated in areas flanking the progressive zone. As a result, focal defects (image blurring) and image distortions occur particularly in the flanking areas of the progressive zone. Unfortunately, the image distortions are manifest as image waviness and other annoying phenomena to a wearer of such lenses when the wearer's visual axis is moved near or within such flanking areas.
Various schemes are manifest in the known progressive power lenses based on differing perspectives on how to solve the problems summarized above.
FIG. 3 schematically illustrates the positional relationships of various portions of a conventional progressive power lens designed symmetrically around a principal meridional curve (such a lens is termed a "symmetrical progressive power lens"). The depicted progressive power lens includes a far-vision portion F located at the upper portion of the lens when the lens is worn, a near-vision portion N located at the lower portion of the lens, and an intermediate-vision portion P located between the far-and near-vision portions. Proceeding downward across the intermediate-vision portion P, the refractive power of the lens progressively changes from the refractive power of the far-vision portion to that of the near-vision portion.
The surface profile of the FIG. 3 lens is defined along a reference line M-M' (termed a "principal meridional curve"). The principal meridional curve M-M' extends on the object-side lens surface and represents a line along which a plane perpendicular to the page intersects the lens. The principal meridional curve M-M' extends (at an angle relative to vertical) from top to bottom at nearly the center of the surface of the lens as worn by the wearer. The principal meridional curve M-M' can be used for expressing the specifications of the lens, such as additional power. In a progressive power lens that has been thus symmetrically configured, the far-vision center OF of the far-vision portion F, the far-vision eye point E, the geometric center OG of the lens surface, and the near-vision center (i.e., the near-vision eye point) ON are all located on the principal meridional curve M-M'.
An "asymmetrical progressive power lens" according to the prior art is shown in FIG. 4. In the FIG. 4 lens, the near-vision portion N is shifted toward the nose side relative to the far-vision portion F and the intermediate-vision portion P. I.e., the near-vision portion N, the intermediate-vision portion P, and the far-vision portion are arranged asymmetrically with respect to each other.
The asymmetrical progressive power lens as shown in FIG. 4 also has a principal meridional curve M-M'. But, rather than being linear as in FIG. 3, the principal meridional curve M-M' in FIG. 4 comprises two lines intersecting at the geometric center OG. The first line represents a first plane (perpendicular to the page) extending through the far-vision center OF of the far-vision portion F, the far-vision eye point E, and the geometric center OG of the lens surface; the second line represents a second plane (perpendicular to the page) extending through the near-vision center ON and the geometric center OG of the lens surface.
In a symmetrical progressive power lens (FIG. 3), the principal meridional curve M-M' symmetrically divides the lens refractive surface into a noseward portion and an earward portion. In an asymmetrical progressive power lens (FIG. 4), in contrast, the principal meridional curve M-M' extending across the intermediate-vision portion P and the near-vision portion N is shifted toward the nose side.
FIG. 5 shows an astigmatic-difference distribution in a conventional symmetrical progressive power lens. In FIG. 5, curves that connect points of equal astigmatic difference, i.e., isoastigmatic-difference curves, are shown.
Generally, the astigmatic-difference value at which a wearer can view an object without experiencing blurred vision is said to be 0.5 diopter (0.5 D) or less. Thus, in FIG. 5, the minimum isoastigmatic-difference curve is a 0.5-D curve. Consequently, objects generally can be viewed without blurred vision within an area including the principal meridional curve and bounded by this 0.5-D isoastigmatic-difference curve. The area in which a wearer can view an object without experiencing blurred vision is termed the "distinct-vision area." The width, in the horizontal direction, of the distinct-vision area is a critical factor in evaluating the performance of a progressive power lens.
The performance of a progressive power lens is evaluated by the maximum width of the distinct-vision area in the far-vision portion F above the far-vision center OF, by the maximum width of the distinct-vision area in the near-vision portion N below the near-vision center ON, and by the minimum width of the distinct-vision area in the intermediate-vision portion P between the far-vision center OF and the near-vision center ON.
A progressive power lens desirably has the following characteristics:
(a) The width of the distinct-vision area is as wide as practicably possible in the far-vision portion F and in the near-vision portion N. PA1 (b) The width of the distinct-vision area is as wide as practicably possible in the intermediate-vision portion P, and the length of the progressive zone along the principal meridional curve is appropriate for the intended use of the lens. (E.g., if the progressive zone is long, eye movement can be undesirably increased when using the lens for close vision, which can cause rotation fatigue. If the progressive zone is short, the distinct-vision area could be too narrow in the intermediate-vision portion P.) PA1 (c) Aberrations are minimized on the refractive surface.
Progressive power lenses typically have a single refractive surface. Because of this, in conventional progressive power lenses, relatively large lens aberrations exist in regions flanking the progressive zone. Such aberrations are generally regarded as theoretically unavoidable in progressive power lenses.
In order for a progressive power lens to be most comfortable for the wearer, the lens should be configured to match the usage requirements of the wearer. Various progressive power lenses are known that match a wearer's specific usage requirements.
For example, FIG. 6 shows typical isoastigmatic-difference curves for a conventional progressive power lens for "everyday use" that emphasizes both far and near vision. (An "everyday-use" lens can be used in many different ways under different conditions by the wearer.) In a lens such as that of FIG. 6, the length of the progressive zone along the principal meridional curve is normally 12 to 15 mm. The angle of ocular rotation is made small (short progressive-zone length), minimizing the amount of movement of the visual axis for close-range vision, while a wide distinct-vision area is maintained in the far-vision portion. Furthermore, a significant distinct-vision area is provided in the near-vision portion and far-vision portion for maximal comfort during use for near vision and far vision.
One of the flaws in conventional progressive power lenses emphasizing both far and near vision is that the width of the distinct-vision area in the intermediate-vision portion is relatively narrow. This results in large lens aberrations in the areas flanking the distinct-vision area. This, in turn, causes substantial image "swim" whenever the wearer's visual axis swings to the flanking areas. Consequently, conventional progressive power lenses emphasizing both far and near vision are suited to usage conditions in which the visual axis does not swing greatly to the flanking areas, e.g., for reading and writing, etc.
FIG. 7 depicts typical isoastigmatic-difference curves for a progressive power lens for everyday use emphasizing far and intermediate vision. In such a lens, the length of the progressive zone along the principal meridional curve is normally 18 mm or more. Also, the width of the distinct-vision area is widest in the far-vision portion. Because the length of the progressive zone along the principal meridional curve is long, the width of the distinct-vision area is also relatively wide in the intermediate-vision portion.
One of the flaws in conventional progressive power lenses emphasizing far and intermediate vision is that they are not suited for close-range work due to the long progressive zone and the narrow distinct-vision area in the near-vision portion. Consequently, conventional progressive power lenses emphasizing far and intermediate vision are suited for usage conditions in which the far-vision portion and the intermediate-vision portion are of primary importance, e.g., participating in sports, etc.
FIG. 8 depicts typical isoastigmatic-difference curves for a conventional "balanced" progressive-power lens for everyday use. In such a lens, the length of the progressive zone is normally 15 to 18 mm. Such a lens exhibits a performance that is between that of a conventional lens emphasizing far and near vision and a conventional lens emphasizing far and intermediate vision. The balanced progressive power lens is the typical type of progressive power lens now being sold. Balanced progressive power lenses are suitable for use in eyeglasses that can be continuously worn for long periods of time, i.e., "extended-wear" glasses.
In any event, with conventional progressive power lenses, a wearer can select from lenses that emphasize far and near vision, far and intermediate vision, or offer a balanced performance. The type usually selected by a particular user is regarded as the most appropriate to his/her usage conditions.
The three types of progressive power lenses discussed above are similar in that they all provide comfortable far vision (e.g., when looking straight ahead at a distant object). This is because far vision is necessary and indispensable in everyday life.
As discussed above, progressive power lenses are equipped with a far-vision portion and a near-vision portion and an intermediate-vision portion comprising a progressive zone in which the refractive power gradually changes, all defined by a single refractive surface. Consequently, when comfort principally for far vision is emphasized by situating a wide distinct-vision area in the far-vision portion, the width of the distinct-vision area of one or both of the intermediate-vision portion and the near-vision portion is conventionally sacrificed to reduce aberrations on the refractive surface as much as possible. This is also true for balanced progressive power lenses.
The three types of progressive power lenses discussed above are generally referred to as "everyday-use progressive power lenses."
As discussed above, progressive power lenses should be designed to meet the specific use requirements of the wearer. Whenever the wearer's use requirement is primarily targeted to close-range work, the progressive power lens worn by the user must be of a type in which the width of the distinct-vision area in the near-vision portion is wider or at least equal to the width of the distinct-vision area of the far-vision portion; i.e., a progressive power lens emphasizing intermediate and near vision. The progressive power lenses disclosed in, e.g., Japan Laid-open Patent Document No. HEI 2-248920 and Japan Patent Publication No. HEI 6-90368 are examples of this type of lens emphasizing intermediate and near vision.
However, whereas prior-art progressive power lenses emphasizing intermediate and near vision provide a somewhat improved vision performance, further improvement is desired. In other words, with the progressive power lens disclosed in Japan Laid-open Patent Document No. HEI 2-248920 emphasizing intermediate and near vision, each of the far-vision portion and the near-vision portion is a separate mathematical point. In addition, the astigmatic difference is distributed over the entire surface area of the lens. As a result, with such a lens, the astigmatic difference of the entire lens may be minimized, but the total astigmatic difference remains, so the distinct-vision areas are small. Widening the distinct-vision areas in the far-vision portion and the near-vision portion using a standard progressive power lens leaves a large part of the astigmatic difference on the lens.
In addition, with the progressive power lens disclosed in Japan Patent Publication No. HEI 6-90368 emphasizing intermediate-near vision, the gradient of refractive power on the principal meridional curve is decreased. Because of this, the position of the far-vision eye point is moved a substantial distance upward, and the position of the near-vision eye point is nearly the same as the corresponding position in a conventional progressive power lens for everyday use. As a result, using such a lens for predominantly close-up work disadvantageously can cause wearer "rotation fatigue" (fatigue from eyeball movement).
In conventional progressive power lenses, it has been impossible in a single lens surface to provide a far-vision portion, a near-vision portion, and the intermediate-vision portion with wide respective distinct-vision areas and with low rotation fatigue, while adequately controlling astigmatic differences.